Alistair and Jonny Brownlee sit at their kitchen table and pause briefly over a dreamy snapshot of the future. In the imagined picture, in the last seconds of a gruelling Olympic triathlon, the brothers are racing towards the finish. The final outcome is in the balance but the current world No1 and 2 will settle the small matter of winning gold and silver medals within the family.

Outside, on a rainy Yorkshire afternoon, the lovely old lady who keeps feeding them lemon drizzle cake has drawn her curtains against the blackness spreading across the village of Bramhope, on the outskirts of Leeds. In their tight little cul-de-sac, and at the bakers down the road where they're also given free treats, the Brownlees are certified celebrities. Yet they would be engulfed by national fame if they could produce a defining Olympic story by finishing first and second or, best of all, in a dead heat in the triathlon next August.

An incredible prospect is not mere fantasy. It is rooted in the reality of their world rankings and the fact that this year they have reached the podium together in five major races where the Olympic standard tests them in a 1.5km swim, a 40km ride and a 10km run. Alistair is the new world champion, for a second time, while Jonny finished just behind him in the world series after the deciding triathlon in Beijing. Over the same course used for the Beijing Games, Alistair's winning time was a minute-and-a-half quicker than that recorded in 2008 by the Olympic champion, Jan Frodeno.

"We have got a good chance," Alistair admits, looking as steely as he is thoughtful. "But everything has to go right for us to finish first and second. In triathlon, there's a 50% chance of something going wrong. So I'd say there is a one in four chance of it happening for both of us. The odds stack up."

Yet, if they were comfortably clear of the field with 50m left, would they consider clutching hands and crossing the line together? "We'll be sprinting hard by then," Alistair says with a dry cackle before Jonny, two years younger than him at 21, completes the sentence: "As if our lives depended on it! Imagine winning Olympic gold at home? It's a once-in-a-lifetime chance so I'll definitely try and outsprint Alistair."

Jonny's big brother grins knowingly. "But, yeah," Alistair says, "it would be an incredible story if we crossed the line together. We were told we weren't allowed to do it in the world series because they'd photo-finish us to see which one of us won. I don't know what would happen if we did it in the Olympics."

Alistair is the slightly more intense member of a friendly pair and his knuckles often rap urgently on the tabletop while Jonny chats away cheerfully. But, strikingly, the older brother lingers over a romantic idea. "Actually, to me," Alistair says of the winning hand‑holding scenario, "it seems very much in the Olympic spirit."

Beyond sibling rivalry, the Brownlees form a canny partnership. The most obvious example unfolded during the European Championships in June, when Alistair's bike suffered a puncture. "I slowed the pack down by physically not doing anything to make them go faster," Jonny says. "So I helped in that small way. But Alistair is a team-mate."

Would he have done the same for Will Clarke – the next best British triathlete? Jonny pauses in surprise, repeating the question out loud, before Alistair answers for him. "Will would probably never be in that position. Jonny and me are the only British athletes who swim at the front and make that breakaway gang."

Alistair laughs when asked if he felt any ambivalence, after Jonny had helped him, in overtaking his brother near the end to become European champion? "No. A little bit – but not much. In Beijing I could have run straight off but I held back for Jonny so I could pull him along. We do small things for each other."

He has also twice slowed down near the finish line so that Jonny could complete a race just a step behind him. "Yeah," Alistair remembers. "I did that in Madrid this summer and last year at the nationals."

The intriguing sub-text is that Jonny could present Alistair with his toughest challenge at London 2012. After we have whipped through the usual comparisons with the Williams sisters and the Klitschko brothers, the Brownlees are momentarily silenced by a trickier analogy. Perhaps they most resemble David and Ed Miliband – with Alistair slotting into the older brother's role of a clear front-runner who could yet be beaten by his supposedly warmer but less heralded sibling?

Last week Alistair had tweeted wryly: "No Spoty nomination but, on a positive note, I am the 39th most influential northerner and only five behind Ed Miliband." Yet, now, Jonny laughs awkwardly at being compared to Ed while Alistair looks mildly unsettled by the suggestion that, just like David, he might suffer a shock defeat to his little brother in a race that will define their lives. Is Alistair secretly worried that Jonny might steal his Olympic glory?

"Er, not really," Alistair parries. "That's the way it goes."

Alistair is acutely aware of how much he improved between the ages of 20, when he led the Beijing Olympics before collapsing through inexperience, and 22. Surely Jonny will improve more than him between now and August?

"Definitely," Alistair says. "Jonny has the age advantage but it's slowing down for him now. Jonny has had three fantastic years where he has been completely injury-free. Since 2008 I've not had a winter without an injury. So if I get a good winter I'll be all right. I might just stay that one step ahead of him."

Alistair says this lightly but it offers a tiny insight into the competitive battle between them – even if, understandably, his brother highlights the "massive advantage" they hold over their rivals in living and training together. Jonny points at the rainy darkness outside their kitchen window. "On a day like today, when it's absolutely pouring and I'm not fancying a run, Alistair will get me out. The next day it might be a case of me getting him out. We train together 95% of the time because we have the same programme. Why not make it easier for each other?"

They are recognised most when training – in a smattering of celebrity that Alistair dislikes. He still shudders when remembering how a brush with fame in Australia quickly wore him down. "The Gold Coast triathlon is really big in Australia and so for a few days I couldn't really walk anywhere without being stopped. I enjoyed coming home to anonymity. But it's changing a little, isn't it?"

Jonny nods. "Yeah, definitely – especially if we're in training gear. We're in the exact same clothes and on the exact same bikes, and so people always look at us and then maybe recognise us. But I'm like Alistair. I don't want to walk down the street and get recognised by everyone. It's a bit of a nightmare in triathlon because it's a bubble and you get all these people watching you as you walk through. To have that all the time, in real life, would be awful."

"There're billboards about, too," Jonny says of the advert, before his brother interrupts. "It's not really you by the time you're on a billboard, is it?" Alistair murmurs.

Both their parents are doctors but it took Alistair just a term at Cambridge to decide that his future lay in triathlon. "I went in with the premise of trying medicine," he says. "I'd got into Cambridge and so I thought I'd better give it a go. But it was always a trial period and I knew instinctively I wanted to do triathlon instead. I'm utterly obsessed with it. I do a bit of uni but it doesn't bother me too much."

Alistair's "bit of uni" includes completing a degree in Physiology and Sports at Leeds University while he is currently a very relaxed MSc student in finance at Leeds Metropolitan. "My academic career is different," Jonny says as a more serious history student at Leeds University. "Alistair can switch off from uni whereas I really try my best. I'm in the midst of a big dissertation on Richard III right now."

Jonny points at his pile of books on the kitchen table: "I was working on it until just before you came. I do enjoy it."

Alistair's absorption in the triathlon is obvious – for it blends technical excellence, physiological endurance and sheer will-power. "I'm totally stimulated by it. I really enjoy reading but a lot of the time, after training, you just want to lie down."

During the last year Alistair has spent most nights in an altitude tent – in an attempt at training while sleeping. But, in another small sign of their rivalry, Jonny breaks out into a big smile. "I've got my own altitude tent being delivered tomorrow. I'm quite excited."

The brothers point to their own beds as being potentially key factors next summer. "It's going to be a very different Olympic experience," Alistair says. "We'll probably stay at home until the Sunday before the race. We'll then go down for the briefing and race on the Tuesday. We'll keep life as normal as possible."

Jonny agrees: "It's going to be a huge help sleeping in our own beds, eating our own food. It's a massive advantage."

The odds on a brotherly one-two seem a little shorter now – even if the London course is not going to present the arduous battle that would suit them most. "If I was choosing my perfect course it would end with a whacking big hill," Alistair says. "It's a shame when you consider that the other Olympic courses have been really iconic and tough. London is just iconic as a city."

London could still provide the backdrop to an extraordinary Yorkshire story built on the iron resilience of two cherubic Tykes. The Brownlees might drool over their plans to one day climb Everest or, in more dynamically competitive conditions, race each other across Europe or over the frozen wastelands of Antarctica. But, more immediately, they could make Olympic history next August.

"We could," Alistair says with a quiet smile before Jonny adds one last telling reminder. "Alistair won the trial event this year in Hyde Park and I was third. So we won't change anything in the months ahead. It's a big challenge but we love it. We live off challenges."